This invention relates to methods for preparing slurries that include depolymerized polysaccharides and depolymerized and derivatized polysaccharides that may be useful in subterranean well operations including fracturing, gravel packing, and frac-packing.
Viscous fluids are added to subterranean formations (such as oil or gas wells), inter alia, to increase their permeability and production. It is believed that these viscous fluids do this by promoting or expanding cracks or “fractures” within the formation. These viscous fluids are called “fracturing fluids,” and may be employed at various times during the life cycle of a production well. Most typically, they are introduced into completed wells penetrating oil or gas-containing formations. They are also used in secondary or tertiary recovery operations to facilitate the injection of other fluids.
Fracturing fluids may be aqueous, or employ organic solvents. These organic solvents include any material useful to form a polymer slurry. Diesel fuel, kerosene, mixed aliphatic oils, alcohols, and alkanes are all known as solvents for these slurries.
Fracturing fluids typically contain a gelling agent such as a polysaccharide. One such polysaccharide known in the art is guar. Guar is typically obtained as a powder, with a mesh size of about 60 to about 400. Native guar has a molecular weight of about 3,000,000. To prepare fracturing fluids, the guar may be “depolymerized,” i.e., broken down into smaller polymer units, having a molecular weight of about 25,000 to about 400,000. This depolymerization is typically carried out before preparation of the fracturing fluid.
For example, guar may be depolymerized by adding guar to a reactor vessel together with a quantity of hydrogen peroxide and water. The reactor vessel is heated to initiate the reaction, which, once it commences, is exothermic. The reactor vessel is maintained at about 100° F. to about 200° F. for a time sufficient for the polymer to degrade to a desired molecular weight. The depolymerized guar may then be added to either an aqueous or organic solvent and may be used in, for example, a fracturing fluid.
Guar can also be “derivatized” by reaction with a variety of chemicals to incorporate desired functionality or capabilities to the guar polymer. Guar may be derivatized before preparation of the fracturing fluid slurry, or the derivatizing agents may be added directly to the slurry to prepare the derivatized guar. Examples of derivatized guars useful in fracturing fluids include carboxymethylguar, carboxymethylhydroxypropylguar, hydroxypropylguar, hydroxyethylguar, carboxymethylhydroxyethylguar, and hydroxybutylguar. Chemical reagents that are known for use in derivatizing guar include alkylene oxides, haloacetic acids such as chloroacetic acid, alkali metal haloacetates, including sodium chloroacetate and sodium bromoacetate. Alkylene oxides yield hydroxyalkyl guar derivatives, while acetate agents produce carboxymethylated guar derivatives. For example, propylene oxide is used to produce hydroxypropyl guar; while chloroacetic acid or sodium bromoacetate may be used to produce carboxymethyl guar. Carboxymethylhydroxypropyl guar may be produced by sequential reaction of guar with propylene oxide and chloroacetic acid.
In addition to the polymer gel, other materials may be added to fracturing fluids to enhance their performance. They may be formulated with viscosifiers, such as organophilic clays. They may also include one or more dispersants, to aid in dispersing or wetting. Surfactants useful for that purpose include alkyl sulfonates, alkyl aryl sulfonates, alkyl ammonium salts, alkyl trimethylammonium chloride, branched alkyl ethoxylated alcohols, phenol-formaldehyde nonionic resin blends, cocobetaines, dioctyl sodium sulfosuccinate, imidazolines, alpha olefin sulfonates, linear alkyl ethoxylated alcohols, and trialkyl benzylammonium chloride, and mixtures thereof.